Currently, an absorbent body, which is constituted by hydrophilic tissue such as pulp and by a water absorbing agent mainly made from acrylic acid (salt) or the like, is put to widespread use in sanitary materials such as disposable diapers, sanitary napkins and incontinence pads. The absorbent body is used for the purpose of absorbing bodily fluids. In recent years, these sanitary materials such as disposable diapers and sanitary napkins have been more advanced and made thinner. Accordingly, there have been increases in (i) the amount of water absorbing agent used for a piece of sanitary material and (ii) a water absorbing agent content in an entire absorbent body constituted by a water absorbing agent, hydrophilic tissue, and the like. That is, research has been conducted on reduction in the thickness of a sanitary material without reducing the amount of water absorption by increasing the ratio of water absorbing agent contained in an absorbent body through (i) using a smaller amount of hydrophilic tissue having small bulk specific gravity and (ii) using a larger amount of water absorbing agent having excellent water absorbency and large bulk specific gravity.
Such a sanitary material, in which a water absorbing agent content is increased by decreasing the ratio of hydrophilic tissue, is preferable from the viewpoint of simply holding a liquid. However, such a sanitary material is rather problematic if distribution and spreading of a liquid when the sanitary material is actually used are taken into consideration.
When absorbing water, a large amount of water absorbing agent per unit volume turns into a softer gel-like form. This induces such a phenomenon as gel blocking which causes considerable reduction in diffusibility of a liquid in a sanitary material. As a result, part of the water absorbing agent, which part is distant from a central area of the sanitary material and is therefore difficult for the liquid to reach, does not effectively function. This prevents the effect of increasing the water absorbing agent content from being sufficiently exerted, and therefore causes an absorbing ability of the sanitary material in actual use to be much lower than a theoretical level.
In order to avoid the problem and maintain absorption property of an absorbent body, a range of the ratio between hydrophilic tissue and a water absorbing agent is inevitably limited. This puts limitations on thinning of sanitary materials.
Examples of an index by which an improvement of gel blocking in a sanitary material is evaluated encompass (i) absorbency against pressure (AAP) or performance under pressure (PUP) and (ii) saline flow conductivity (hereinafter abbreviated as “SFC”; see Patent Literature 1 and the like).
Examples of a well-known technique for improving gel blocking encompass a technique in which crosslinking densities inside and outside of a water absorbing agent are changed by surface treatment (see Patent Literatures 2 to 21). There are also known attempts to improve water absorbing ability, particularly liquid diffusibility, by combining surface treatment with, as a liquid permeability enhancer, (i) inorganic fine particles, (ii) inorganic compounds such as polyvalent metal salt, or (iii) cationic polymer compounds (see Patent Literatures 22 to 35). Furthermore, a technique for controlling a reaction environment of surface crosslinking treatment is also known (see Patent Literatures 36 to 39).
However, while the aforementioned well-known methods can prevent gel blocking, the methods also pose the following problems: (i) Liquid diffusibility, particularly saline flow conductivity (hereinafter also referred to as “SFC”) and gel bed permeability (hereinafter also referred to as “GBP”), in a sanitary material does not attain desired performance. (ii) Even though SFC and GBP are sufficient, absorption capacity (CRC) and absorption capacity under load (AAP or PUP) are extremely lowered. (iii) In actual production of a water absorbing agent, performance of the water absorbing agent cannot be maintained sufficiently. For example, excellent properties of a water absorbing agent cannot be consistently obtained during production.